Get the Facts: Recent Canadian Mammography Study Flawed, but How?

What happens when “flawed” studies get media attention? Lives are at stake—which is why we turn to the experts to help us determine what’s right and wrong. If you’ve heard about a recent Canadian study (known as the Canadian National Breast Screening Study [CNBSS]) debating the benefits of mammography published in the British Medical Journal (BMJ), you must learn the facts. After the results of the study began getting media coverage, a number of rebuttals were issued by credible sources, including industry experts, the American College of Radiology (ACR), and the Society of Breast Imaging (SBI).


This “new study”  is actually a follow up on a very old Canadian study which had already been widely discredited in the US and around the world. According to world famous Dr. László Tabár and Tony Hsiu-Hsi Chen, the Canadian trial stands apart as the only one among many published mammography screening trials that failed to show any decrease in death rates from breast cancer. All the other major trials have shown significantly decreased deaths in patients asked to have screening mammograms. More from Dr. László Tabár:



Why did the Canadian study fail to show benefit?  Experts have pointed out many reasons, from flaws in the study’s design to deficiencies in quality and technology.


One of the flaws in the study is that the women involved in the study were not all randomly selected. Before assigning the women to be in the group offered screening or in the control group of women, investigators first did physical exams of the breasts and underarm areas. There have been claims that women who had suspicious physical exams were placed in the “mammography arm” of the trial. This would explain why the mammography group had more advanced cancers than the women in the control (non-mammography) group. Obviously, it would be hard to show benefit from mammography if the deck was stacked with more breast cancers in the mammography group—especially more advanced cancers which led to deaths..


Finally, the equipment used in the study included older, second hand mammography units, considered by many to be substandard. According to the statement from the ACR and SBI, the quality of the images was degraded by “scatter,” resulting in cloudy images. This was because the researchers did not employ grids for much of the trial. Images were also impaired by poor positioning, including a view of the breast which is no longer used. The radiologists in the Canadian study had no specific training in mammographic interpretation. The CNBSS’ own physicist expressed concerns about the quality of mammography in some of the study’s screening centers, even for the early 1980s when the study was conducted. It’s no wonder that fewer early cancers were found.


So what should women do with the information from this recent study? Dan Kopans, MD, FACR, FSBI, of Harvard University sums it up best:

[Women] should ignore this information. The trial was poorly designed and executed and the data are corrupted. Analysts need to be honest and stop citing this trial as having any validity.

“The death rate from breast cancer had been unchanged for 50 years. Mammography screening began at a national level in the U.S. in the mid 1980′s and, as expected, the death rate began to fall soon after. There are now 30% fewer deaths from breast cancer each year than would have occurred without screening. Opponents of screening do not treat women with breast cancer. [Cancer] therapy saves lives when cancers are treated earlier.”


Posted in American Cancer Society, Breast Cancer Detection Guidelines, Diagnostic Mammography, Screening Mammography

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